SummarySalmonellin production on agar was reduced by adding streptomycin or raising the incubation temperature to 44°. Production of group 3b salcol was increased by incubation at 44° and, for one producer only, by adding streptomycin. Production of the diffusible, nontransferable group 4 salcol and also of diffusible colicin V was reduced at 44°. Group 1 salcol production was unaffected by raised temperature or streptomycin. Group 2 salcol production also was unaffected, except in salm+ transcipients of Arizona So116 where it was reduced along with salmonellin production by incubation at 44° or addition of streptomycin.